1. Field of the Invention
This invention relates to a reproducing apparatus for plural types of optical discs whose recording densities are different from each other.
2. Description of the Prior Art
When information is recorded on an information recording medium as a digital data, recording is carried out such that the digital signal indicative of the information is modulated as digital data by a specific modulation method selected from various types of modulating methods. In this respect, if the information recording medium is an optical disc, the situation is the same as that of the above-mentioned case. For, example, in an optical disc which is known as a compact disc (CD), recording and reproducing are carried out by modulation using a modulation method (EFM) such that eight data bits are enlarged to fourteen channel bits. In the CD for which the above-mentioned EFM is adopted, digital data is formed on the surface of the CD by a bit train of a modulated signal including nine types of bit lengths which are successively set to either one between the modulated signal of bit length (pulse length or pulse width ) 3 T corresponding to the minimum interval Tmin between bits and a bit length of 11 T corresponding to the maximum interval Tmax between bits.
In the above-mentioned recording/reproducing method, digital data including nine types of bit lengths which are successively set to either one between the modulated signal of the shortest bit length 3 T and a longest bit length of 11 T is recorded on the optical disc (CD) as an array of pits. Reproducing (reading out) the information recorded in accordance with the condition of pits arranged on the CD is carried out by projecting and focusing a reproducing light beam (read out light) having a specific wavelength 870 nm) on a CD through a condenser lens (objective lens) with a specific numerical aperture (NA=0.45). When the recorded information from the above-mentioned CD is reproduced, the reproducing light is projected on the array of pits provided on the signal plane of the CD such that a portion of a beam waist of the reproducing light condensed by the condenser lens positions on a signal plane of the CD, i.e., the signal plane of the CD positions at a focus point of the condenser lens. In -this condition, a reproduced signal is obtained by photoelectric conversion of the reproduced light, reflected and diffracted from the signal plane of the CD in this condition.
The above-mentioned reproduced signal, i.e., the reproduced signal obtained in the condition that a light spot having a given diameter is focused onto the signal plane of the CD so positioned at the focus point of the condenser lens, is supplied to a comparator to which a voltage value which is about half of an amplitude of the reproduced signal is applied as a comparator reference voltage (threshold voltage) in order to perform demodulation operation. This causes demodulation of digital data which comprises the bit train of a modulated signal including nine types of bit lengths which are successively set to either one between the modulated signal of bit length (pulse length or pulse width) 3 T corresponding to the minimum interval Tmin between bits and a bit length of 11 T corresponding to the maximum interval Tmax between bits.
Then, to demodulate, with high fidelity, the digital data recorded in the condition that various conditions specified in the specification of the CD are satisfied, it is necessary for demodulation that: as mentioned above, the reproducing light (read out light) having a specific wavelength (780 nm) through -a condenser lens (objective lens) with a specific numerical aperture (NA=0.45) is focused on a CD; and the reproducing light is projected and focused on the array of pits provided on the signal plane of the CD such that a portion of a beam waist of the reproducing light condensed by a condenser lens positions on a signal plane of the CD, in other words, the signal plane of the CD positions at a focus point of the condenser lens. In this condition, in order to demodulate, the reproduced signal obtained from a photodetector is applied to the comparator to which the voltage value which is about half of an amplitude of the reproduced signal is applied. For example, if the wavelength of the reproducing light or the numerical aperture of the condenser lens used in the reproduction apparatus deviates from the values specified in the specification of the CD or a voltage value of the reference voltage (threshold voltage) of the comparator used for demodulation of the reproduction signal deviates over the tolerance from the voltage value which is about half of the amplitude of the reproduce signal, it may be happen that the digital data comprising the bit train of the modulated signal including nine types of bit lengths which are successively set between the modulated signal of bit length (pulse length or pulse width) 3 T corresponding to the minimum interval Tmin between bits and a bit length of 11 T corresponding to the maximum interval Tmax between bits, cannot be demodulated correctly. Thus, for example, the modulated signal of 3 T bit length corresponding the shortest interval Tmin between bits, which is recorded on the CD may be demodulated as a modulated signal having a bit length other than a bit length of 3 T.
In the reproducing apparatus of the CD, as the modulated signal of the bit length of 3 T corresponding to the shortest interval Tmin between bits should be demodulated as a demodulated signal of 3 T bit length for example, in order to obtain the demodulated signal having the same bit length as that recorded on the CD. Thus, the reproducing light beam (read out light) having the specific wavelength (780 nm) through a condenser lens with the specific numerical aperture (NA=0.45) is focused on a CD; a reproduced signal is obtained in the condition that a light spot with a given diameter is focused on the array of pits provided on the signal plane of the CD; and the reproduced signal is applied to the comparator to which a voltage value (threshold voltage) which is about half of an amplitude of the reproduced signal is applied in order to perform demodulation operation.
Then, in order to correctly demodulate the modulated signal recorded on the optical disc on which information is recorded as the array of pits formed on the signal plane of the optical disc, as clearly described in the above description of the CD mentioned as an example, it is necessary that the diameter of spot of the reproducing light to be used on reproducing of the information signal recorded on the optical disc ranges in the specific values specified in correspondence with the size and form of the pit; and that the comparator reference voltage (threshold voltage) used for demodulating the reproduced signal is the voltage value of about half of the amplitude of the reproduced signal, etc.
For the reasons mentioned above, as a reproducing apparatus for plural types of optical discs capable of correct demodulation of the information signal recorded on plural types of optical discs having different recording densities, the reproducing apparatus needs different structures in either or both of a light source of the reproducing light and the condenser lens used, that is, such reproducing apparatus requires different optical heads having light sources of the reproducing light capable of emitting light having different wavelengths and condenser lenses having different numerical apertures respectively.
Then, put the above-mentioned point will be more specifically as follows:
Because the size of the pit formed on the signal plane of the optical disc becomes smaller with the higher the recording density generally in the case that the recording density is increased but the rotational speed of the optical disc is remained constant, in the reproducing apparatus for obtaining the reproduced signal using diffraction phenomenon of the reproducing light reflected at pits by projecting and focusing the reproducing light onto the signal plane, the spot of the reproducing light having a diameter with a given relation according to an width of the pit in the radial direction of the optical disc to be reproduced is projected and focused onto the signal plane of the optical disc.
Moreover, conditions of array of pits formed on the signal plane of the optical disc are different from each other in accordance with the recording density of the optical disc though recording/reproducing is performed for modulation of digital data by the same modulating method for example. In the case that optical discs having different recording densities are used and driven at different rotational speeds, the pit of an optical disc having a higher recording density has pits whose width is smaller than that of an optical disc having a lower recording density. As mentioned, conditions of pit arrays formed on the signal planes of optical discs are different from each other with recording densities of optical discs.
Then, because the condition of the light diffraction at the pit portion is totally different naturally if pits are read out with the spot having the same wavelength and the same diameter in the case that the sizes of pits of optical discs are different with each other, it is clear that the reproduced signals outputted from the photodetectors are different from each other.
In order to resolve the above-mentioned problem that the diffraction state of light produced by pits changes with change in relation between the width of the pit and the diameter of the spot used for reading out the pit, it may be considered that a cross-sectional area of the reproducing light on the signal plane of the optical disc is changed with change in condition of condensing of the reproducing light. However, wave front of the reproducing light at the signal plane of the optical disc is disturbed because a cross section of the reproducing light projected onto the signal plane of the optical disc is not the beam waist portion. Therefore, though cross section of the reproducing light at the signal plane of the optical disc is changed in accordance with the above-mentioned resolution method, that is, though the diameter of the reproducing light at the signal plane of the optical disc is formed to have the specific diameter, the reproduced signal cannot be reproduced correctly from the optical disc. Therefore, as a reproducing apparatus for producing plural types of optical discs capable of correct demodulation of the information signal recorded on plural optical discs having different recording densities, the reproducing apparatus needs different read heads in either or both of a light source of the reproducing light and the condenser lens used in the structure of the reproducing apparatus.
However, because it is undesirable that in one reproducing apparatus, different read heads are provided for respective types of optical discs having different recording densities, it was considered that a reproducing apparatus used for an optical disc, having a specification that a diameter of a spot of the reproduced light projected on the signal plane of the optical disc in focus on reproducing is smallest was tried to be used for reproducing other type optical discs in common.
However, it is obvious that a correctly demodulated signal cannot be obtained though the voltage value of about half of the amplitude of the reproduce signal as the comparator reference voltage (threshold voltage) on demodulation of the reproduced signal is used unless a spot of the reproducing light having a diameter with a given relation according to an width of the pit on the optical disc is projected onto the signal plane of the optical disc in the reproducing apparatus for obtaining a reproduced signal using diffraction phenomenon of reproducing light coming from pits by projecting the reproducing light onto the signal plane. Further, there is also a problem that it is possible that the comparator reference voltage (threshold voltage) cannot be set which is necessary for demodulation of the reproduced signal to obtain a correct demodulated signal. Therefore, a resolution to these problems is required.
The above-mentioned point will be described more specifically. It is assumed that one of plural optical discs having different recording densities is a CD wherein reproducing light of 780 nm wavelength and a condenser lens of 0.45 numerical aperture as mentioned above are used. Also it is assumed that another optical disc capable of higher recording density than the CD is reproduced using light of 670 nm wavelength as reproducing light and a condenser lens of 0.50 numerical aperture. When a reproducing apparatus using the reproducing light of 670 nm wavelength and the condenser lens of 0.50 numerical aperture, i.e., a reproducing apparatus for reproducing the optical disc having a recording density which is higher than that of the CD reproduces a CD, the reproduced signal from the reproducing apparatus has an eye pattern shown in FIG. 5.
Then, in the case that the reproduced signal as shown in FIG. 5 is supplied to the comparator to which the voltage value of about half of the amplitude of the reproduced signal as a comparator reference voltage (threshold voltage) for demodulation operation, that is, in the case that a comparator reference signal (threshold voltage) of the line S--S shown in FIG. 5 is used to demodulate, the digital data comprising the bit train of the modulated signal including nine types of bit lengths which are successively set to either one between the modulated signal of bit length (pulse length or pulse width) 3 T corresponding to the minimum interval Tmin between bits and a bit length of 11 T corresponding to the maximum interval Tmax between bits, cannot be demodulated correctly. In FIG. 5, a comparator reference voltage (threshold voltage) capable of correct demodulation of the digital data comprising the bit train of the modulated signal including nine types of bit lengths which are successively set to either one between the modulated signal of bit length (pulse length or pulse width) 3 T corresponding to the minimum interval Tmin between bits and a bit length of 11 T corresponding to the maximum interval Tmax between bits is a voltage shown by the line A--A in FIG. 5 when the reproduced signal is that one shown in FIG. 5. As clearly shown by the waveform of the reproduced signal in FIG. 5, a stable demodulated signal cannot be obtained at the line A--A. Therefore, the reproducing apparatus used for the optical disc defined by a specification such that a diameter of the reproducing light projected onto the signal plane of the optical disc in focus in the reproducing modes smallest cannot be used for other types of optical discs having different recording densities in common.